1, three points that are not on a straight line determine a circle.

2. The circumscribed circle of the triangle: the circle passing through the three vertices of the triangle.

3. Exterior center of triangle: the intersection of the perpendicular lines of the three sides of triangle, that is, the center of the circumscribed circle of triangle.

4. The inscribed circle of a triangle: a circle tangent to all three sides of the triangle.

5. Heart of triangle: the intersection of bisectors of three angles of triangle, that is, the center of inscribed circle of triangle.

In a plane, a closed curve formed by rotating a certain length around a certain point is called a circle. On the plane, a circle is a set of points whose distance from a fixed point is equal to a fixed length.

The circle is defined as 360, because the ancient Babylonians moved one position every 4 minutes and 360 positions every 24 hours, so the inner angle of the circle is defined as 360. This degree represents the sun.

Triangle is a closed figure composed of three line segments on the same plane but not on the same straight line, which has applications in mathematics and architecture.

Ordinary triangles are divided into ordinary triangles (three sides are unequal) and isosceles triangles (isosceles triangles with unequal waist and bottom and isosceles triangles with equal waist and bottom, that is, equilateral triangles); According to the angle, there are right triangle, acute triangle and obtuse triangle, among which acute triangle and obtuse triangle are collectively called oblique triangle.